1. Field of the Invention
The invention relates to the preparation of polyimide-epoxy thermoset resins. More specifically, it relates to the two-step reaction of a polyepoxide first with a solution of a polyimide dianhydride and then subsequently with a solution of a polyimide diamine. The polyimide dianhydride used has a relative anhydride reactivity of at least 0.17 and the polyepoxide is used in an amount giving a ratio of epoxy equivalents to anhydride plus amine equivalents of at least 1/1. Still more specifically, a two step reaction is used which offers a more tractable polyimide-epoxy intermediate resin that can be processed into void-free products with superior mechanical strength than previously obtained polyimide-epoxy thermoset resins. Even more specifically, this invention relates to such polyimide-epoxy resins suitable for use as coatings, and the preparation of laminates therefrom suitable for various high temperature applications.
2. State of the Prior Art
The industrial applications of polyepoxides cured with agents such as cyclic anhydrides and polyamino compounds have been known since the early 1950's (see H. Lee and K. Neveille Ed. "Handbook of Epoxy Resins", McGraw-Hill, Inc., 1967, Chapters 1 and 5, and the references therein).
The amine-epoxy addition reaction requires the presence of a hydrogen donor. The primary amine can usually react with two epoxy groups if the resulting secondary amine derived from the primary amine-epoxy reaction does not possess too much steric hinderance. However, the tertiary amines generated by the reaction of two epoxy groups with a primary amine are known to be ineffective catalysts for further epoxy reactions. It is also known that the cyclic anhydride-epoxy reaction in the presence of a catalyst can proceed at temperatures as low as 25.degree. C., even though more moderate temperatures such as 60.degree.-80.degree. C. or higher are usually employed. The reaction of cyclic anhydride with epoxy groups usually starts with the formation of a half ester, a carboxylic acid and a nascent hydroxyl group. The carboxylic acid and the nascent hydroxyl group can react further with the excess epoxy groups through esterification and etherification. The reaction of carboxylic acid with the epoxy groups usually starts at moderate temperatures such as 80.degree. C.-100.degree. C. or higher, whereas the etherification requires not only higher temperatures, that is above 120.degree. C., but also a high epoxy/anhydride ratio and strongly acidic conditions. When a Lewis base is present, the etherification can be excluded completely even at temperatures greater than 120.degree. C., even up to 150.degree. C.
The thermal stability of cyclic anydride or polyamino compound cured polyepoxide can be expected to increase if the thermal stability of the curing agent is increased by using thermally stable moieties for carrying the required reactive functionality. One of the most logical moieties of this kind is a reaction product of an aromatic diamine with an aromatic dianhydride, i.e., an aromatic polyimide moiety. The compatibility between the aromatic polyimides and the polyepoxides is generally very poor due to the large difference of their solubility parameter or cohesive energy density. Since the curing reactions of epoxy with anhydride and epoxy with amine usually start at temperatures of 60.degree. C. to 160.degree. C. which are well below the softening temperature of the aromatic polyimides, which is generally above 200.degree. C., the reaction of the polyepoxide with unfused polyimide-anhydride or polyimide-amine will usually form a thermoset coating on the particulated polyimide and prevent further epoxy-dianhydride or epoxy-amine reaction inside the polyimide particles. It is thus easy to see that even if the degree of polymerization of the anhydride-end capped or amine end-capped polyimide is equal to 3, it is almost impossible to make any well reacted copolymeric product of thermoset polyimide-epoxy resin without the advantage of a good solvent for both the polyepoxide and the polyimide compounds.
It has been found that many of the polyimide diamines and polyimide dianhydrides are not sufficiently soluble for suitable reaction between the polyepoxide and the functional groups in the polyimide compound. It is considered that a polyimide compound having a solubility in a particular solvent of less than 5% by weight is "insoluble" for the purpose of this invention. As described hereinafter, it is possible to improve the solubility of such insoluble polyimide components so that they can react favorably with polyepoxides. For preparing laminates and coatings it is generally desirable to have concentrations of 20% or more.
U.S. Pat. No. 3,663,651 implies that it is possible to make a thermoset PIM-Epoxy resin by reacting polyepoxide with some polyimide dianhydrides derived from pyromellitic or benzophenone-tetracarboxylic dianhydride. Throughout the teaching and working examples of this patent, there is no teaching of a process for forming a soluble reaction polyimide product in the proclaimed solvent systems, such as dimethylformamide (DMF) or N-methyl-2-pyrrolidone (NMP) and an anhydride activity above 0.17. Furthermore, it has been found that when any polyimide dianhydride is prepared using DMF as solvent, there is no detectable anhydride absorption peak (1840 cm.sup.-1) in the product's infrared spectrum whereas an identical preparation in phenol or m-cresol gives a product with a substantial absorption at 1840 cm.sup.-1.
The reason for this failure to produce polyimide dianhydrides with high anhydride reactivity (I.sub.R) when these amide solvents are used is not known, but it is suspected that there is a reaction occurring between the anhydride groups and these solvents during imidization at high temperatures. As has been found, a complete imidization in these solvents usually requires temperatures above 160.degree. C. (but still below 170.degree. C.) for several hours. It is quite possible that the DMF, as well as related solvents containing n-alkylated amide groups react with the anhydride groups. This results in intermediates which have low anhydride activity, herein defined as I.sub.R. However, whatever the reason, it has been found impossible to prepare well reacted polyimide-epoxy thermoset resins from a polyimide dianhydride which is prepared in DMF or a related solvent under prior art known processes.
U.S. Pat. No. 4,277,583 describes the reaction of amine-terminated polyimides with polyepoxides. While reference is made to the fact that these reactions may be performed in the presence of a solvent in which the aromatic polyimide is soluble, Examples VII-XI, XIII, XIV and XVI show coreaction of intimate mixtures of the solid reactants. Example XII describes the reaction as conducted in a DMF solution containing 21.38 g. of amine-terminated polyimide in 75 ml. of DMF together with 3.4 g. of Epon 828. Example XVI is a repetition of Example XII using a different polyepoxide. There is no teaching in this reference of how to conduct a reaction of a polyepoxide in solution with a polyimide component which is insoluble in the particular solvent being used. This is particularly important because of the more thorough and complete reaction effected in solution and also because of the number of polyimide diamines and dianhydrides which are insoluble or poorly soluble. Moreover there is no teaching of a subsequent reaction with a solution of a polyimide diamine after a preliminary reaction with a polyimide dianhydride.
A problem encountered when an anhydride-terminated polyamic acid is used for preparing a polyimide-epoxy thermoset laminate is that delamination occurs when the molding or post-curing temperature is above 180.degree. C. However, since the use of high molding temperatures of this kind (above 180.degree. C.) are required to complete cyclization to imide groups and a complete curing of polyimide-epoxy thermoset resin, such laminates produced from the polyamic type are unsatisfactory. Imidization of a polyamic acid in a good solvent requires temperatures of 120.degree.-160.degree. C. with the final few percent of imidization in the solid state requiring temperatures above 160.degree. C. It is thus possible that trapped volatile product, such as H.sub.2 O from imidization, introduces flaws or creates delaminations during molding of these laminates or during later applications when the laminate is being exposed to temperatures above 160.degree. C.
It is important therefore, that the method, including the solvent used for preparing the polyimide dianhydride, is one that produces these intermediates with a high anhydride activity (I.sub.R). The high I.sub.R, good solubility and low fusion temperatures are desirable for good subsequent reaction with a polyepoxide.
In this field of polyimides there are a number of terms which are commonly used, such as "degree of polymerization" (DP), "molar ratio of monomers" (r.sub.m), "statistical average of structure reoccurrence" (n), "degree of imidization" (C), "relative reactivity" (I.sub.R), and the "ratio of epoxy equivalents to anhydride equivalents". These are defined as follows:
The Molar Ratio of starting monomers is represented as r.sub.m or X/Y, with X representing moles of diamine and Y the moles of dianhydride.
Degree of Polymerization (DP)--Polyimides may be prepared by reacting X moles of diamine with Y moles of dianhydride. To produce an anhydride-terminated polyimide, Y is greater than X. The statistical average "degree of polymerization" (DP) may be calculated on the basis that the formation of the intermediate amic acid groups is completed by the relatively long reaction periods used (at least 3 hours) as compared to the relatively short time for amic acid formation (about 30 minutes). Therefore: EQU DP=(1+r.sub.m)/(1-r.sub.m)
Statistical Average of Structure Reoccurrence (n) is equal to: EQU (DP-1)/2=r.sub.m /(1-r.sub.m)
For example, where r.sub.m is 0.5 and DP is 3, then n is 1.
Degree of Imidization (C) is equal to the amount of water distilled from the reaction divided by the amount of water theoretically to be removed by complete imidization. This is equal to (2n.times.18) grams for making one gram mole of polyimide.
Relative Reactivity (I.sub.R) is the ratio of the intensity peak ratio of the absorption peak of the anhydride group at 1840 cm.sup.-1 to that of the imide group at 1790 cm in the Infrared Spectrum of the polyimide.
Equivalents Ratio of epoxy to anhydride (R) is: R=(No. of equivalent weights of polyepoxide)/(No. of equivalent weights of dianhydride) wherein the number of equivalent weights of a component is the weight of the component divided by the equivalent weight of the component.
With regard to U.S. Pat. No. 3,663,651, repetition of its working examples gives no detectable anhydride absorption peak (1840 cm.sup.-1) in its infrared spectrum. This means that the anhydride activity (I.sub.R) is practically zero. As discussed above, this is believed to be because of reaction between the anhydride groups and DMF.
In producing thermoset products of good properties from polyimide dianhydrides and polyepoxides, it is found to be important that the polyimide dianhydride has an anhydride activity (I.sub.R) of at least 0.17 and that it is soluble to the extent described herein. Many of the anhydride-terminated or amine-terminated polyimides do not have sufficient solubility to give satisfactory reaction with polyepoxides for production of void-free molded products and good laminated products. For this purpose it is considered that preferred minimum solubility is that in which one gram of the polyimide is completely dissolved in 4 ml. of the solvent when heated at 165.degree. C. with occasional stirring for 5 minutes.